Noble metals such as Pt are used as chemical catalysts instead of being used in accessories. For example, the noble metals are used in an exhaust emission control system of a vehicle and a solid polymer fuel cell, for example. Particularly, a methanol type solid polymer fuel cell using a methanol solution as a fuel can be operated at low temperature, and, at the same time, since the size and weight are small, the methanol type solid polymer fuel cell has been actively studied recently for the purpose of being applied to a power supply of a mobile apparatus and the like.
Thus, it has been considered to add other elements to PtRu for the purpose of improving methanol oxidation activity. In a solution technique such as a dipping method generally used in catalyst synthesis, a metal desired to be supported is precipitated on a surface of carbon fine particles in a solution to be oxidized once. Thereafter, the oxide is reduced and returned to metal. Accordingly, although heat treatment under a reduction atmosphere is required, the heat treatment temperature significantly varies depending on elements. Pt and Ru often used generally can be reduced at approximately the same temperature, and an alloy of them is easily formed.
However, when an element improving a catalyst activation level is to be contained, some elements can be reduced by raising a temperature to be much higher than a temperature at which the element is reduced to support Pt. Further, at that time, there are many elements that react with carbon as a supporting base material.
Thus, a catalyst synthesis method under vacuum using a sputtering method or an evaporation method is considered. In this method, since a desired element can be directly evaporated to carbon of a supporting base material, reduction treatment is not required to be performed, and PtRu can be easily alloyed at room temperature.
However, in the conventional sputtering method or evaporation method, catalyst fine particles can be supported only on carbon processed into a sheet (hereinafter referred to as “carbon paper”), and the thickness of a catalyst layer is approximately several μm. In a methanol type fuel cell, although a fuel dispersed into anode is oxidized by a catalyst layer, if the catalyst layer is thin, the amount of methanol that is passed through the catalyst layer without being oxidized is increased. Methanol passed through the catalyst layer without being oxidized permeates a proton-conducting membrane separating an anode and a cathode to reach the cathode, and, thus, to generate water by the catalyst of the cathode. When a large amount of water is generated in the cathode, the cathode is clogged with water, and the cathode cannot play an essential role, that is cannot decompose oxygen in the air and generate an oxygen radical, whereby the output is lowered.
Since the catalyst obtained by being sputtered directly on the carbon paper is adhered as a film to a surface of a fiber constituting the carbon paper as shown in FIG. 21, an active surface area is small, and there is a problem that a high output cannot be obtained.
Although water is essential at an anode, if the catalyst layer is thin, the amount of water required for power generation cannot be held in the catalyst layer, so that a satisfactory performance may not be obtained. Thus, although it is considered to increase the thickness of the catalyst layer, if the thickness of the catalyst layer is to be increased, the catalyst layer becomes not fine particles but a film-like shape. Therefore, the surface area of the catalyst is small, and there is a problem that the power generation performance is reduced.
Thus, there has been attempted a method of growing a carbon fiber on the carbon paper by a CVD method and forming a catalyst thereon by the sputtering method. However, when a long carbon fiber is grown, the catalyst is adhered only to the vicinity of the front end of the fiber as shown in FIG. 22. Consequently, since the effective catalyst adhesion area is reduced, a sufficient amount of catalyst cannot be supported, and a high power generation performance cannot be obtained. Further, in the carbon fiber grown by the CVD method, since the mechanical strength and the adhesion strength with a base material are weak, there is such a problem that the catalyst is easily dropped in the process after the catalyst is physically evaporated.
There is disclosed a technique of growing a carbon nanotube with a length of about 10 nm to about 10 mm on a carbon nanofiber substrate and depositing catalyst metal particles such as Pt thereon (for example, see Patent Document 1).
There is further disclosed a technique of sequentially forming a first catalyst layer, an electrolyte layer containing carbon particles and the like, second catalyst layer, and the like on a proton-conducting membrane (for example, see Patent Document 2).